The present disclosure relates to devices and methods that permit stabilization of the bony elements of the skeleton. The devices and methods permit adjustment and maintenance of the spatial relationship(s) between neighboring bones.
Spinal disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. The current surgical treatment of abnormal vertebral motion and low back pain is the complete immobilization and bony fusion of the involved spinal segment. An extensive array of surgical techniques and implantable devices has been formulated to accomplish this goal.
The growing experience with spinal fusion has shed light on the long-term consequences of vertebral immobilization. It is now accepted that fusion of a specific spinal level will increase the load on the spinal segments immediately above and below the fused level. Further, as a consequence of fusion, each adjacent disc will experience a displaced center of rotation and produce an aberrant motion profile. The increased load and abnormal movement experienced by the adjacent discs will synergistically act to accelerate the rate of degeneration at these levels. Consequently, the number of patients who require extension of their fusion to the adjacent, degenerating levels has increased with time. This second procedure necessitates re-dissection through the prior, scarred operative field and carries significantly greater risk than the initial procedure while providing a reduced probability of pain relief. Further, extension of the fusion will increase the load on the motion segments that now lie at either end of the fusion construct and will accelerate the rate of degeneration at those levels. Thus, spinal fusion begets additional, future fusion surgery.
There is a growing recognition that segmental spinal fusion and complete immobilization is an inadequate solution to degenerative disc disease. Replacement of the degenerated and painful disc with a mobile prosthesis is a more intuitive and rational treatment option. This approach preserves spinal mobility in a majority of spinal segments and reserves fusion and complete immobilization for those disc spaces where the degenerative disease is advanced and beyond surgical restoration.
U.S. Pat. Nos. 4,759,769; 4,997,432; 5,674,294; 5,674,296; 5,676,701; 5,888,226; 6,001,130; 6,019,792; 6,162,252; 6,348,071; 6,368,350; 6,419,706; 6,520,996; 6,540,785; 6,607,558; 6,645,249; 6,673,113; 6,749,635 and others have illustrated various artificial disc prosthesis. Despite the number of proposed designs, each device is sized to substantially occupy the majority of the disc space and replace the entire disc. Since the neural elements are anatomically positioned immediately posterior to the disc space, these large devices can be implanted only through an anterior or lateral surgical approach.
The spine is situated at the most posterior aspect of the body cavities and it can be most readily reached through a posterior approach. Anterior and lateral surgical approaches must dissect around and through the many vital organs and blood vessels that lie anterior to the spine and these approaches add to the risk and morbidity of the procedure. In addition, spine surgeons are more familiar with and technically versed in the posterior approach, further increasing the risks of the more difficult non-posterior approaches. Finally, the posterior approach allows the surgeon to advantageously remove the bone spurs that compress the neural elements at the same time they access the disc space.
The use of a posterior surgical approach to implant a mobile disc prosthesis has numerous advantages. Unfortunately, the intervening nerve elements limit the size of the posterior corridor that can be used to access the anterior disc space and a posteriorly-placed mobile disc prosthesis (i.e. “artificial disc”) must be small enough to fit within that limited implantation corridor. Consequently, a posteriorly-placed artificial disc can only provide partial coverage of the disc space and partial replacement of the inter-vertebral disc. Attempts to overcome this problem by placing several implants within the disc space is limited by the significant difficulty in producing coordinated movement of separate implants about a specified center of rotation.